Sathyabama University
CHAPTER 4
CORROSION SCIENCE
Introduction: Definition. Types: Dry corrosion: Mechanism – Pilling – Bedworth rule – Wet
corrosion: Mechanism. Types: Galvanic corrosion and differential aeration cell corrosion.
Galvanic series and its significance. Factors influencing corrosion. Corrosion prevention:
Material selection and design – Cathodic protection. Protective coatings: Paints –
Constituents. Mechanism of drying of drying oils.
4.1 INTRODUCTION
Most of the modern day industrial as well as domestic applications involve the use of metals
and alloys. A serious problem associated with every use of metal object is corrosion.
Corrosion is a general term that refers to the deterioration and ultimate destruction of a metal
due to its reaction with the surrounding gaseous or liquid environment. Corrosion occurs with
all metals except the noble metals like gold and platinum. Corrosion can also occur in
materials other than metals, such as glass, wood, concrete and plastics. In these cases the
term degradation is more commonly used than corrosion. The term corrosion is exclusively
associated with the surface degradation of metals. The most familiar example of corrosion is
rusting of iron and steel. It is the largest problem encountered throughout the world, mainly
due to the fact that the iron and steel are the most abundantly used materials. It is estimated
that about 20% of iron produced annually are used just to replace the iron objects that have
been discarded due to corrosion.
4.2.1 DEFINITION
Corrosion is defined as “the destruction or deterioration or loss of metallic materials through
chemical or electrochemical attack by the surrounding environment”. The process of
corrosion in reality is the transformation of pure metal into its undesired metallic compounds.
The life of a metallic object gets shortened by the corrosion process and also loses its useful
properties such as malleability, ductility, electrical conductivity and optical reflectability. The
familiar examples of corrosion include:
(i) Rusting of iron – a reddish brown scale formation on iron and steel objects due to the
formation of hydrated ferric oxide.
(ii) Green scale formed on copper vessels – it is due to the formation of basic cupric
carbonate (CuCO3 + Cu(OH)2).
4.2.2 CAUSE OF CORROSION
Most of the metals, except noble metals exist in nature in the combined form such as oxides,
carbonates, sulphides, chlorides, silicates etc. The metals are generally extracted from the
ores by reduction process. During the extraction of metals from their ores a considerable
amount of energy is supplied in the form of heat or electrical energy. Hence the pure metals
are considered to be in the higher energy state compared to their corresponding ores. They
have a natural tendency to revert back to their combined state i.e. lower energy state.
Therefore, when metals are put to use in various forms, they combine with the constituents
of the environment and get converted into their compounds. Thus corrosion of metals can be
considered as the reverse of extraction of metals.
Corrosion ( oxidation )

→ Metallic compounds + Energy
Metal ←

Extraction of metals ( reduction )
Page 1
Sathyabama University
4.2.3 GRAVITY OF CORROSION
The losses incurred due to corrosion are very high and cannot be estimated by considering
only the metal loss. The indirect losses like fabrication cost and cost involved in preventing
the corrosion also have to be taken into consideration. The corrosion in metal objects like
equipments, instruments, chemical plants, structures etc. may make them inefficient,
ineffective, and useless and their unobserved defects due to corrosion may cause their
failure. In such cases, the indirect losses may be much higher than the direct losses. It is
estimated about two lakh crore worth losses are incurred due to corrosion of steel every year
in India. The release of inflammable gases or poisonous gases from the corroded
equipments leads to fire accidents or atmospheric pollution.
The secret of effective engineering lies in controlling rather than preventing the corrosion
because it is impracticable to eliminate corrosion. This can be successfully done only if we
know the mechanism of corrosion and methods of control mechanisms. This chapter
highlights some of the important facts about corrosion and its control.
4.3 TYPES OF CORROSION
Corrosion can be broadly classified into two types based on the mechanism of corrosion.
a) Dry or direct chemical corrosion
b) Wet or electrochemical corrosion
4.3.1 DRY OR DIRECT CHEMICAL CORROSION
This type of corrosion mainly occurs through the direct interaction of atmospheric gases
such as oxygen, halogens, hydrogen sulphide, sulphur dioxide, oxides of nitrogen or
anhydrous liquid with metal surfaces.
When a metal is exposed to a gas, it adsorbs the gas and slowly reacts with it and forms the
corrosion product. The corrosion product may be insoluble, soluble or a liquid. In the first
case when it is insoluble, a solid film of the corrosion product is usually formed on the
surface of the metal which protects the metal from further corrosion. However, if a soluble or
a liquid corrosion product is formed, then the metal is exposed to further attack.
There are three main types of chemical corrosion.
1. OXIDATION CORROSION
Oxygen is mainly responsible for the corrosion of most metallic structures as compared to
other gases and chemicals. This type of corrosion is brought about by the direct chemical
action of oxygen at low or high temperature on metals generally in the absence of moisture.
Alkali and alkaline earth metals (e.g., Na, Ca, Mg etc.) suffer extensive oxidation even at low
temperatures, where as at high temperatures, practically all metals except Ag, Au and Pt are
oxidized.
MECHANISM
When a metal surface is exposed to a gas such as oxygen at room temperature the gas gets
adsorbed physically. The gas molecules are attracted to the metal surface by vander Waals
forces of attraction. When the temperature is increased, electrons are transferred from the
metal to oxygen resulting in the formation of metal ions and oxide ions simultaneously. The
metal ions and the oxide ions combine together to form the metal oxide.
M→M2+ + 2e–
1/2O2 + 2e–→ O2-
Page 2
Sathyabama University
The overall reaction may be represented as,
M + 1/2O2→M2+ + 2e–
Figure 4.1 Oxidation corrosion
The formation of metal oxide film at the surface actually tends to restrict further oxidation.
For oxidation to continue either the metal ions must diffuse outwards through the scale to the
surface or the oxide ions must diffuse inwards through the scale to the underlying metal.
Both transfers occur (fig. 4.1) but the outward diffusion of metal ions is likely to be more
rapid due to the fact that cations are smaller in size compared to the oxide ions.
NATURE OF THE OXIDE
When oxidation starts, a thin layer of oxide is formed on the metal surface and the nature of
this film decides the further action. This film may be
(i)Stable – A stable layer is fine grained structure and can adhere tightly to the parent metal
surface. Hence such a layer can cut off penetration of attacking oxygen to the underlying
metal. Such a film is protective in nature thereby shielding the metal surface. The oxide films
on Al, Sn, Pb, Cu, Cr etc. are of stable, hence further oxidation corrosion is prevented.
(ii)Unstable – The oxide layer formed decomposes back into the metal and oxygen.
Metal oxide
→
Metal + Oxygen
Consequently, oxidation corrosion is not possible in such a case. Thus Ag, Au and Pt do not
undergo oxidation corrosion.
Page 3
Sathyabama University
Figure 4.2 Unstable oxide layer
(iii)Volatile – The oxide layer volatalizes as soon as it is formed, thereby leaving the
underlying metal surface exposed for further attack. This causes rapid and continuous
corrosion. E.g. Molybdenum oxide (MnO3) is volatile.
Figure 4.3 Volatile oxide layer
(iv)Porous – The oxide film formed has pores or cracks. In this case, the atmospheric
oxygen has access to the underlying surface of metal, through the pores or cracks of the
layers thereby the corrosion continues unobstructed until the entire metal is completely
converted into its oxide. E.g. Li, K, Na, Ca, Mg etc.
Fig
Figure 4.4 Porous oxide layer
PILLING AND BEDWORTH RULE
The protective and non protective nature of the oxide layer can be predicted on the basis of
Pilling – Bedworth rule.
According to this rule an oxide layer is protective or non – porous, “if the volume of the oxide
is at least as great as the volume of the metal from which it is formed” and the oxide layer is
porous or non – protective “if the volume of the oxide is less than the volume of the metal
from which it is formed.” For example, heavy metals such as Al, Cr, Pb, Sn etc. form a
protective oxide film and prevent further corrosion. Whereas metals like alkali (Li, Na, K etc.)
and alkaline earth metals (Be, Ca, Sr etc.) form porous oxide films, which allow further
diffusion of oxygen to come in contact with the fresh metal surface and results in severe
corrosion.
Page 4
Sathyabama University
2. CORROSION BY OTHER GASES
The other gases present in the environment such as chlorine, fluorine, sulphurdioxide,
hydrogen sulphide, carbon dioxide etc. are also corrosive. The extent of corrosion of a
particular metal depends on the chemical affinity of the metal and the gas involved. The
degree of attack depends on the formation of protective or non – protective films on the
metal surface.
For example chlorine attacks tin and forms volatile SnCl4 and hence the metal is
continuously exposed to the corrosive gas and undergoes rapid corrosion. In contrast a non
– porous protective film of AgCl is formed on silver due to attack of chlorine decreases the
further corrosion.
Hydrogen sulphide attacks metals forming corresponding metal sulphide and release atomic
hydrogen. The hydrogen attacks the metal in two ways.
HYDROGEN EMBRITTLEMENT
At room temperature the atomic hydrogen diffuse readily into the metal and collects at the
void spaces, where the atomic hydrogen combines to form hydrogen gas. The accumulation
of the gas develops a high pressure causing cracks and blisters in the metal. This condition
is known as hydrogen embrittlement.
DECARBURIZATION
At higher temperatures molecular hydrogen undergoes dissociation to yield atomic
hydrogen. The active atomic hydrogen attacks the steel and combines with the carbon
present in the steel to form methane gas which collects in the voids. As the pressure
increases due to the accumulation of the gas, cracks occur in steel, a condition known as
decarburization. As a result of decarburization the metal loses some of its strength and
ductility.
C +4H → CH4
3. LIQUID METAL CORROSION
It is due to the chemical action of flowing liquid metal at high temperature on a solid metal or
alloy. This type of corrosion occurs in devices used for nuclear power. The corrosion reaction
involves either the dissolution of the solid metal or internal penetration of the liquid metal into
the solid metal.
4.3.2 WET OR ELECTROCHEMICAL CORROSION
Most of the corrosion problems are best explained on the basis of electrochemical corrosion.
These are often called wet corrosion since aqueous medium or moist air is required for
corrosion to take place.
This type of corrosion occurs due to the formation of anodic and cathodic areas on the same
metal surface or when two different metals or alloys are in contact with each other in the
presence of conducting medium. At the anodic area oxidation reaction takes place and the
metal gets converted into its ions, liberating electrons. Consequently, metal undergoes
corrosion at the anodic area.
At anodic area,
M → Mn+ + ne–
Page 5
Sathyabama University
Mn+ → Dissolves in the solution
Forms compounds such as oxides
On the other hand at the cathodic area, reduction takes place. Since the metal cannot be
reduced further some of the constituents present in the corrosion medium take part in the
cathodic reaction to form anions like OH-, O2-. Hence the metal atoms at the cathodic areas
are unaffected by the cathodic reaction
The electrons liberated at the anodic area migrate to the cathodic area constituting corrosion
current. The metal ions liberated at the anode and some anions (like OH-, O2-) formed at the
cathode diffuse towards each other through the conducting medium and form corrosion
product somewhere between the anode and the cathode. Corrosion of metal continues as
long as both anodic and cathodic reactions, take place.
MECHANISM OF WET OR ELECTROCHEMICAL CORROSION
Electrochemical corrosion involves flow of electron – current between the anodic and
cathodic areas. Anodic reaction is a simple oxidation reaction in which metal atoms are
converted into their ions liberating electrons.
At the anodic area:
M → Mn+ + ne–
For example when iron undergoes corrosion,
Fe → Fe2+ + 2e–
On the other hand the cathodic reaction consumes electrons with either by (a) evolution of
hydrogen or (b) absorption of oxygen depending on the nature of the corrosive environment.
Correspondingly the corrosion types are known as hydrogen type corrosion and oxygen type
corrosion.
(a) Evolution of hydrogen
This type of corrosion occurs in acidic medium and in the absence of oxygen. For example in
the case of iron metal, the anodic reaction is dissolution of iron as ferrous iron with the
liberation of electrons.
Fe → Fe2+ + 2e–
(oxidation)
These electrons flow through the metal from anode to cathode, where H+ ions (of acidic
solution) are reduced to form H2 gas.
2H+ + 2e– → H2 ↑
(reduction)
The overall reaction is,
Fe + 2H+→ Fe2+ + H2 ↑
Page 6
Sathyabama University
Figure 4.5 Mechanism of wet corrosion by hydrogen evolution
This type of corrosion is generally considered as the displacement of hydrogen ions from the
acidic solution by the metal ions. The metals above hydrogen in the electrochemical series
have a tendency to get dissolved in acidic solution with simultaneous evolution of hydrogen.
In this type of corrosion anodes have large areas whereas cathodes are small areas.
(b) Absorption of oxygen
This type of corrosion occurs in neutral or alkaline medium and in the presence of oxygen.
Hydrogen ion concentration is very low in neutral or alkaline solution, thus evolution of
hydrogen gas is not favorable. In such cases, reduction of oxygen to hydroxyl ion occurs.
The surface of iron is usually coated with a thin film of iron oxide. The small cracks in the
oxide film create small anodic areas while the rest of the part acts as a cathode.
Figure 4.6 Mechanism of wet corrosion by oxygen absorption
At the anodic areas the metal (iron) dissolves as ferrous ions with the liberation of electrons.
Fe → Fe2+ + 2e–
Page 7
Sathyabama University
The liberated electrons flow from anode to cathode through iron metal, where electrons are
intercepted by the dissolved oxygen as,
1/2O2 + H2O + 2e– → 2OH–
The Fe2+ ions (at anode) and OH- ions (at cathode) diffuse and when they meet, ferrous
hydroxide is precipitated.
Fe2+ + OH– → Fe(OH)2
Since smaller Fe2+ ions diffuse more rapidly than OH- ions, their combination occurs more
commonly near the cathodic area. The corrosion occurs near the anodic area and the
corrosion product deposits near the cathodic area.
If enough oxygen is present, ferrous hydroxide is easily oxidized to ferric hydroxide.
4Fe(OH)2 + O2 +H2O → 4Fe(OH)3
This product is called yellow rust and it is chemically Fe2O3.H2O.
If the supply of oxygen is limited the corrosion product may be black anhydrous magnetite,
Fe3O4. Generally, presence of oxygen greatly accelerates both corrosion and rust formation.
COMPARISON BETWEEN CHEMICAL AND ELECTROCHEMICAL CORROSION
S. No
Chemical corrosion
Electrochemical Corrosion
1.
It occurs in the dry state
2.
It is due to direct chemical attack
of the metal by the environment.
Follows adsorption mechanism
It takes place in the presence of
moisture or electrolyte
It is due to the formation of large
number of small galvanic cells.
Follows electrochemical reaction
mechanism
Surface heterogeneity or bimetallic
contact is necessary for this type
of corrosion
Corrosion occurs at the anode and
the product settles near the
cathode. So the Corrosion is not
self controlled.
Pitting corrosion is likely to occur
when the anode is small in area
Examples
Rusting
of
iron
in
moist
atmosphere and dezincification of
brass
3.
4.
Even a homogeneous
surface will be corroded
5.
Corrosion products accumulate in
the same place where corrosion
occurs. So the corrosion is often
self controlled
Uniform corrosion occurs.
6.
7.
metal
Examples
Rusting of iron in dry state,
reaction of dry HCl gas on Fe and
tarnishing of Ag in air containing
H2S
Page 8
Sathyabama University
TYPES OF ELECTROCHEMICAL CORROSION
4.3.3. GALVANIC CORROSION
Galvanic corrosion (also called “dissimilar metal corrosion”) occurs when two different metals
have physical or electrical contact with each other and are immersed in a common
electrolyte. In a galvanic couple, the more active metal (called the anode) corrodes at a
faster rate and the more noble metal (called the cathode) corrodes at a slower rate. For
galvanic corrosion to occur, three conditions must be present. They are
1. Electrochemically dissimilar metals must be present
2. The metals must be in electrical contact, and
3. The metals must be exposed to an electrolyte
Figure 4.7 Galvanic corrosion
Figure 4.7 represents Zn-Fe bimetallic couple. Of the two metals, zinc (more active)
dissolves at a faster rate compared to iron (less active). Here zinc act as the anode and
undergo corrosion, whereas, iron act as cathode and is protected from corrosion. Similarly, a
bimetallic couple can be set up with iron and copper in which, iron (more active) undergo
corrosion at a faster rate whereas Cu (less active) is protected from corrosion.
Factors such as relative size of anode, types of metal and operating conditions (temperature,
humidity, salinity, etc) affect galvanic corrosion. The surface area ratio of the anode and
cathode directly affects the corrosion rates of the materials. Galvanic corrosion is often
utilized in sacrificial anodes.
4.3.4 DIFFERENTIAL
CORROSION
AERATION
CORROSION
OR
CONCENTRATION
CELL
Corrosion of metals arising as a result of the formation of an oxygen concentration cell due
to the uneven supply of air on the metal surface is known as differential aeration corrosion. It
occurs when a metal surface is exposed to differential air concentrations or oxygen
concentrations. The part of the metal exposed to higher oxygen concentration acts as
Page 9
Sathyabama University
cathodic region and part of the metal exposed to lower oxygen concentration acts as anodic
region. Consequently, poorly oxygenated region of the metal undergo corrosion.
At the anode (less oxygen concentration), metal undergoes corrosion.
M → Mn+ + ne–
At the cathode (more oxygen concentration), oxygen is reduced.
1/2O2 +H2O + 2e– → 2OH–
There are different types of differential aeration corrosion. Some examples are pitting
corrosion, pipeline corrosion, corrosion in wire fence and water line corrosion.
4.3.5 PITTING CORROSION
Pitting corrosion is a localized corrosion that occurs on a metal surface. The pits are often
found underneath surface deposits caused by corrosion product accumulation. Pitting is
considered to be more dangerous than uniform corrosion damage as it is more difficult to
detect, predict and design against. A small, narrow pit with minimal overall metal loss can
lead to the failure of an entire engineering system.
Figure 4.8 Pitting corrosion
Pitting corrosion can be explained from the figure shown above.
When a drop of water or NaCl solution is placed on a metal surface, the area below the drop
of water, which does not have much access to oxygen, becomes anodic with respect to the
other areas, which are freely exposed to air.
The area exposed to air demand more electrons for reduction from the smaller area covered
by the drop of solution. Therefore, more and more material is removed from the smaller
anodic area, which results in the formation of a hole or a pit. Pit formed is now more away
from the atmosphere which act as better anode and corrosion proceeds at a very fast rate.
At anode the metal (iron) undergoes oxidation:
Fe → Fe2+ +2e–
Page
10
Sathyabama University
At cathode, oxygen undergoes reduction:
1/2O2 + H2O + 2e– → 2OH–
Pitting is initiated by,
1. Localized chemical or mechanical damage to the protective oxide film
2. Localized damage to, or poor application of, a protective coating
3. Presence of non-uniformities in the metal structure of the component, e.g.
nonmetallic inclusions.
Stainless steels are sensitive to pitting corrosion, but other metals, such as passive iron,
chromium, cobalt, aluminium, copper and their alloys are prone to this form of damage.
4.3.6 MICROBIOLOGICALLY INFLUENCED CORROSION (MIC)
Micribiologically influenced corrosion, also called bacterial corrosion, bio-corrosion,
microbiologically-induced corrosion or microbial corrosion is caused by specific genera of
bacteria which feed on nutrients and other elements found in waters and soils. Sea water is
a primary source of sulphate reducing bacteria (SRB). The biological activities modify the
local environment and render it more corrosive to the metals. For example, iron-oxidizing
bacteria can perforate a 5mm thick stainless steel tank in just over a month.
MIC is not caused by a single microbe, but is attributed to many different microbes. These
are often categorized by common characteristics such as by-products (i.e., sludge
producing) or compounds they affect (i.e., sulfur oxidising). In a general sense, they fall into
one of the two groups based upon their oxygen requirements; on being aerobic (requires
oxygen) such as sulfur oxidizing bacteria, and the other being anaerobic, (requires little or no
oxygen), such as sulfate reducing bacteria. MIC can cause various forms of localized
corrosions such as pitting corrosion, de-alloying, stress corrosion, cracking and hydrogen
embrittlement etc. The wide variety of bacteria associated with MIC can cause problems in
almost any type of residential, commercial or industrial setting.
MIC operates as an individual nodule covering a pit. The development of this process occurs
in three phases, which are:
1. Attachment of microbes
2. Growth of nodule and initial pit
3. Mature pit and nodule.
Once MIC is established in a system it is quite difficult to eliminate. There are several ways
to test MIC and one of the most nondestructive methods is to simply determine the types
and quantities of microbes present in a system.
Many industries such as chemical processing industries, nuclear power generation, onshore
and offshore oil and gas industries and underground pipeline industries are affected by MIC.
Page
11
Sathyabama University
Mechanism
Under favourable conditions, bacteria such as SRB reduce inorganic sulphates to sulphides.
When these compounds come into contact with iron (buried pipelines and pipe from deep
water wells) in the presence of moisture, it gets converted to rust and FeS.
Na2S + 4H2O +2Fe → Fe(OH)2 + FeS + 2NaOH
Prevention
MIC can be prevented through a number of methods:
1. Regular mechanical cleaning
2. Chemical treatment with biocides to control the population of bacteria
3. Complete drainage and dry-storage
4.4 FACTORS INFLUENCING THE RATE OF CHEMICAL AND ELECTROCHEMICAL
CORROSION
The type, speed, cause and seriousness of corrosion depends upon the following factors
that can be classified based on the
1. Nature of the metal and
2. Nature of the environment
4.4.1
NATURE OF THE METALS
a) Electrode potential or position in EMF series
Metals with higher electrode potentials do not corrode easily. They are noble metals like,
gold, platinum, silver. Metals with lower electrode potentials, readily undergo corrosion e.g.
metals like zinc, magnesium, aluminium. When two metals are in contact with each other,
higher the difference in electrode potentials greater is the corrosion. For e.g., the potential
difference between iron and copper is0.78V which is more than that between iron and tin
(0.3V). Therefore, iron corrodes faster when in contact with copper that with tin. On this
account, the use of dissimilar metals should be avoided wherever possible.
b) Hydrogen overvoltage
The metal with lower hydrogen over voltage on its surface is more susceptible for corrosion,
when cathodic reaction is hydrogen evolution type. Since, at lower hydrogen over voltage,
liberation of hydrogen gas is easy, cathodic reaction occurs at a very fast rate. This in turn,
makes the anodic reaction(corrosion) also to proceed at a faster rate. Thus, higher the over
voltage lesser is the corrosion.
c) Ratio of cathodic to anodic region
The rate of corrosion is influenced by relative size of cathodic to anodic area. If the metal
has small anode and large cathodic region, corrosion rate is very high. As the ratio inreases
corrosion rate further increases. This is because at anode electrons are liberated and are
consumed at cathodic region. If the cathodic region is larger, the liberated electrons are
rapidly consumed at cathode. This further enhances the anodic reaction leading to an
increase in the overall rate of corrosion.
Page
12
Sathyabama University
d) Nature of the corrosion product
The corrosion product (like metal oxide) formed act as protective film, if it is stable, insoluble,
and nonporous. If it acts as protective film it prevents further corrosion by acting as a barrier
between the metal surface and the corrosion medium. On the other hand, if the corrosion
product is unstable, porous, and soluble it further enhances the corrosion.
e) Purity of metals
Hundred percentage pure metals will not undergo any type of corrosion. Presence of
impurity creates heterogeneity in the metal resulting in the formation of large number of
distinct galvanic cells at different parts of the metal leading to corrosion of the metal. Higher
the percentage of impurity higher will be the rate of corrosion.
4.4.2. NATURE OF THE ENVIRONMENT
a) Temperature
Rate of corrosion increases with the increase in temperature. This is due to the increase in
the conductance of the medium with increase in temperature and hence an increase in the
diffusion rate. Consequently, corrosion progresses faster at higher temperatures. In some
cases, rise in temperature decreases passivity, which again leads to an increase in the
corrosion rate.
b) Humidity
Rate of corrosion will be high when the relative humidity of the environment is high.
Presence of moisture enhances electrochemical type of corrosion.
c) Suspended solids
Solid pollutants in the environment such as NaCl, dirt, dust, charcoal increases the rate of
corrosion.
d) Impurities in the environment
Gaseous impurities such as SO2, CO2, H2S which makes the environment acidic increases
the rate of corrosion of metals.
e) pH of the corrosive environment
Rate of corrosion is higher in acidic pH than in the neutral or alkaline pHs. In the case of
iron, at very high pH protective coating of iron oxide is formed which prevents corrosion,
whereas, at low pH severe corrosion takes place. For metals like aluminium, corrosion rate
is high even at high pH values.
4.5 GALVANIC SERIES
In the electrochemical series, a metal which is present above is more active than the metal
which is present below. Though this series gives useful information about the chemical
reactivity of metals, it does not provide any information in predicting the corrosion of metals.
For example, in Zn – Al couple, zinc which is below aluminium in the electrochemical series
is corroded while aluminium is protected from corrosion. In view of this, the electrode
potential of various metals and their alloys was measured using Calomel electrode as the
reference electrode and immersing the metal or alloy in sea water.
Page
13
Sathyabama University
The galvanic series is ‘the arrangement of all the metals and alloys in the decreasing order
of their activity’.
Active
(or Anodic)
1. Mg
2. Mg alloys
3. Zn
4. Al
5. Cd
6. Al alloys
7. Mild steel
8. Cast iron
9. High Ni cast iron
10. Pb – Sn solder
11. Pb
12. Sn
13. Iconel
14. Ni – Mo – Fe alloys
15. Brasses
16. Monel metal (7% Ni, 30% Cu. Rest Fe)
17. Silver solder
18. Cu
19. Ni
20. Chromium stainless steel
21. 18 – 8 stainless steel
22. 18 – 8 Mo stainless steel
Noble
(or cathodic)
23. Ag
24. Ti
25. Graphite
26. Au
27. Pt
SIGNIFICANCE OF GALVANIC SERIES
Because of the limitations of the Emf Series for predicting galvanic relations, and also
because alloys are not included, the Galvanic Series has been developed.
1. The potentials that determine the position of a metal in the Galvanic Series may
include steady - state values in addition to truly reversible values; hence, alloys and
passive metals are included.
Page
14
Sathyabama University
2. Some metals occupy two positions in the Galvanic Series, depending on whether
they are active or passive, whereas in the Emf Series only the active positions are
possible.
3.
Although only one Emf Series exists, there can be several Galvanic Series with
various environments and differences in tendency to form surface films. In general,
therefore, a specific Galvanic Series exists for each environment, and the relative
positions of metals in such Series may vary from one environment to another.
4. When two dissimilar metals are coupled together the corrosion depends not only on
how far apart they are in the Galvanic Series (potential difference), but also on their
relative areas and the extent to which they are polarized.
5. The potential difference of the polarized electrodes and the conductivity of the
corrosive environment determine how much current flows between them.
4.6 CORROSION PREVENTION
Corrosion can be controlled in so many ways. The basic methods of protection of a metal
from corrosion are
(i) Material selection and design
(ii) Cathodic protection
(iii) Use of inhibitors
4.6.1. MATERIAL SELECTION AND DESIGN
Corrosion in metals can be prevented right from the initial step of making equipments like
choosing the appropriate metal which does not undergo corrosion and framing a suitable
structure for the equipment.
Material Selection
The first and the foremost method of controlling corrosion is the selection of right type of
material. The material selection should be made not only on its cost and structure but also
on its chemical properties and its environment.
1. Noble metals are more immune to corrosion. But they cannot be used for general
purpose since they are costly.
2. The next choice is the selection of pure metal. Impurities in a metal cause
heterogeneity which increases the rate corrosion. Even the presence of minute amount
of impurities may cause severe corrosion. For example minute quantities of Fe in Mg or
Pb in Zn leads to extensive corrosion.
3. Alloying improves corrosion resistance e.g. Presence of Chromium in stainless steel.
It produces an exceptionally coherent oxide film which is self healing and protects steel
from corrosion.
4. Heat treatment like annealing helps to reduce internal stresses and reduces
corrosion.
5. If an active metal is selected, it should be insulated from more cathodic metal.
Page
15
Sathyabama University
Designing
The equipment should be designed in such a way that even if corrosion occurs it should be
uniform and should not be intense and localized. The following conditions may be followed
during designing of the equipments to reduce the rate of corrosion.
1. Two dissimilar metals should not be in contact with each other. Otherwise the more
active metal gets corroded and the less active metal is protected from corrosion.
2. If the contact between two dissimilar metals is unavoidable, then the metals should
be selected in such a way that their oxidation potentials are as near as possible i.e.
they should be as close as possible to each other in the electrochemical series.
3. If the contact between two dissimilar metals is unavoidable, then the anodic area
should be very large when compared to the cathodic area.
4. Moisture should be removed whenever possible. If moisture or electrolytic solution is
present, suitable inhibitors should be added.
5. When two dissimilar metals have to be in contact, then they should be separated by a
suitable insulating material.
6. A proper design should not contain crevices between two adjacent parts since this
causes differential aeration corrosion.
Figure 4.9 Designs with and without crevices
7. A proper design should allow the adequate cleaning of the critical parts. For example,
if sharp corners or recesses are present, there may be accumulation of dirt or
impurities which causes heterogeneity. Hence a proper design should have smooth
joints and bents to avoid the accumulation of impurities.
Figure 4.10 Designing of materials with smooth joint and bends
Whenever possible, the equipment should be supported on legs to allow free circulation
of air.
Page
16
Sathyabama University
Figure 4.11 Design having free circulation of air
4.6.2. CATHODIC PROTECTION
When two dissimilar metals are coupled together, the more active metal acts as anode and
undergoes corrosion. The less active metal acts as cathode and is protected from corrosion.
Hence, anode always undergoes corrosion. Cathodic protection is forcing a metal to act as
cathode so that corrosion does not occur in this base metal. There are two ways by which
cathodic protection is carried out.
1. Sacrificial anodic protection
2. Impressed current cathodic protection
SACRIFICIAL ANODIC PROTECTION
Sacrificial anodic protection involves the protection of a metal at the expense of some other
more active metal. The more active metal acts as sacrificial anode and protects the less
active metal. This method is generally used for the protection of underground cables or
pipes.
In this method, a plate or block of a reactive metal is buried beside the iron pipe or tank
which is to be protected from corrosion and both are connected by a wire. Since the more
reactive metal has a greater tendency to get oxidized, it undergoes oxidation in preference to
iron. The more active metal thus acts as anode and is called ‘sacrificial anode’. Metals
commonly used as sacrificial anodes are magnesium, zinc, aluminium and their alloys.
Magnesium has the most negative potential and is widely used in high resistivity electrolytes
such as soil. Zinc is used as anode in good electrolytes like sea water.
Mg → Mg2+ + 2e–
The electrons migrate to the iron object which starts acting as cathode. At iron cathode,
electrons reduce oxygen into OH- ions.
1/2O2 + H2O + 2e– → 2OH–
Page
17
Sathyabama University
Figure 4.12 Sacrificial anodic protection
The anode gradually disappears due to oxidation. The iron object remains protected as long
as the sacrificial anode is present. The corroded anode block is replaced by a fresh one,
when corroded completely. Important applications of sacrificial anodic method are protection
of buried pipeline, underground cables, marine structures, ship hulls etc.
IMPRESSED CURRENT CATHODIC PROTECTION
Whenever there is corrosion, because of the redox reaction (oxidation of anode and
reduction at the cathode), a current is produced which is called corrosion current. In this
method, an impressed current is applies in the opposite direction to convert the anode to
cathode and to nullify the corrosion current.
The metal structure to be protected is connected to the negative terminal of a DC source like
a battery or a rectifier. The positive terminal of the DC source is connected to an inert anode.
The inert anodes used for this purpose are graphite, high silica iron, platinised titanium etc.
The anode is usually buried in a backfill composed of a mixture of gypsum, bentonite and
sodium sulphate. This increases the electrical contact with the surrounding soil.
Figure 4.13 Impressed current method
This method is useful for the protection of large underground structures like water tanks,
buried oil or water pipes, transmission line towers, marine pipes etc. However, this method
involves large installation and maintenance cost.
Page
18
Sathyabama University
4.6.3. CORROSION INHIBITORS
Corrosion inhibitors are ‘chemical substances which when added in small quantities to the
aqueous or gaseous corrosion environment, retard or slow down the corrosion processes
resulting in the reduction of overall corrosion rate’. Corrosion inhibitors are broadly classified
two types.
1. Immersion inhibitors
2. Atmospheric inhibitors
IMMERSION INHIBITORS
They are used in solutions to achieve inhibition. There are two types of immersion inhibitors.
(1) Anodic inhibitors
(2) Cathodic inhibitors
ANODIC INHIBITORS
Examples of anodic inhibitors are alkalis, chromates, phosphates, molybdates etc. The basic
reaction at the anodic area is oxidation i.e. formation of metal ions.
M → Mn+ +2e–
When the anodic inhibitors are added to the corrosive environment, they react with the metal
ions from the anodic area and form a sparingly soluble salt which gets adsorbed at the
surface of the anodic area. This forms a protective film or barrier on the anode, which
prevents the oxidation of the metals. This reduces the rate of corrosion.
CATHODIC INHIBITORS
(a) In acidic medium, the basic cathodic reaction is evolution of hydrogen gas.
2H+ +2e– → H2 ↑
Rate of corrosion may be reduced either by slowing down the diffusion of hydrogen ions
through the cathode or by increasing the hydrogen over voltage. The diffusion of hydrogen
ions can be reduced by adding organic inhibitors like amines, pyridine, mercaptans,
substituted urea and thiourea, quinoline etc. These positively charged cationic groups attach
themselves to the cathodic areas through the nitrogen and provide inhibition. The efficiency
of inhibition depends upon the size and the number of alkyl groups. For example, primary
amyl amine (C5H11NH2) has been found to be a more effective inhibitor than primary ethyl
amine (C2H5NH2).
The hydrogen over voltage can be increased by adding antimony oxide, arsenic oxide or
sodium meta arsenite. These substances deposit an adherent film of metallic antimony or
arsenic at the cathodic areas, thereby considerably increasing the hydrogen over voltage.
(b) In neutral aqueous medium, the cathodic reaction is,
1/2O2 + H2O + 2e– → 2OH–
The formation of OH- ions is only due to the presence of oxygen. Hence, corrosion can be
controlled either by removing oxygen from the environment or by preventing the diffusion of
Page
19
Sathyabama University
oxygen to the cathodic area. Oxygen can be removed from the solution by adding
substances like sodium sulphide, sodium sulphate or hyrdazene. The diffusion of oxygen
towards the cathodic area is prevented by adding inhibitors like magnesium, zinc or nickel
salts which react with the hydroxyl ions at the cathode to form insoluble hydroxides which
deposit on the cathode. These deposits are impermeable barriers and hence retard the
diffusion of oxygen at the cathode.
VAPOUR PHASE INHIBITORS
These are organic inhibitors which readily vapourise and form a protective layer of the
inhibitor on the metal surface. These are conveniently used to avoid corrosion in enclosed
spaces and also during storage, packing, shipping etc. Cyclohexylamine carbonate,
benzoate, Dicyclohexylamine nitrite most widely used vapour phase inhibitors.
4.7 PROTECTIVE COATINGS
Due to modern technology there is an increase in stress on the metals. The use of metals
increases and also the stress on the metals. Stress leads to corrosion and this can be
prevented by many methods which are discussed in the previous sections. Some of the
methods are costly. New alloys with improved properties can prevent corrosion. But even
alloys undergo stress corrosion. Hence protective coatings plays an important role in
preventing corrosion.
Protective coating is coating a metal to be prevented from corrosion with another metal or an
inorganic material or an organic material. This coating forms a physical barrier between the
surface of the metal and the environment thereby protecting the metal from corrosion.
Organic coating is one of the important protective coatings. Examples of organic coating are
paints, varnishes and lacquers.
Paints
Paint is a viscous, opaque solution consisting of one or more pigments in a medium or
vehicle. Vehicle (drying oil) is a non-volatile film forming material dissolved in a suitable
volatile solvent called thinner.
Ingredients of a paint
A paint essentially consists of the following ingredients.
(i) Pigment, (ii) Vehicle or drying oil, (iii) Thinner, (iv) Drier,
Plasticiser and (vii) Antiskinning agent.
1. Pigments
They are finely ground colouring matters.
Functions : To give colour and opacity to the paint.
Examples:
Black Blue Brown -
Lamp black
Prussian blue
Burnt umber
Page
20
(v) Filler or extender, (vi)
Sathyabama University
Green
Red
Yellow
White
-
Chrome green
Red lead
Chrome yellow
Zinc oxide
2. Vehicle or Drying oil
The vehicle is a non-volatile liquid and film forming material which holds all the ingredients of
a paint together.
Functions: (i) To spread the paint evenly on the surface. (ii) To provide a binder for the
ingredients of a paint so that they may stick or adhere to the surface.
Examples: Linseed oil, Tung oil.
3. Thinner
It is a highly volatile solvent.
Functions :
(i) To make the paint thin so that it can be easily applied on the surface.
(iii) To dissolve the vehicle.
(iv) (iii) To suspend the pigments.
Examples: Turpentine, Toluol, Xylol, Kerosene, etc.
4. Driers
These are the substances used to accelerate the process of drying.
Functions: To accelerate the drying of drying oil through oxidation, polymerization and
condensation.
Examples: Litharge, Red lead, (Resinates and linoleates of Co, Mn and Pb).
5. Extenders or Fillers
Functions: (i) To bring down the cost of paint. (ii) To improve the durability of paint. (iii) To
prevent shrinkage and cracking.
Examples : Barytes (BaSO4), Gypsum, China clay.
6. Plasticisers
Sometimes they are added to paint to provide elasticity to the film and to minimize its crack.
Examples : Triphenyl phosphate, Tributyl phthalate.
7. Antiskinning Agents
They are added to the paint in order to prevent gelling and skinning of the finished product.
Examples : Poly hydroxy phenol.
Pigment Volume Concentration (P.V.C.)
Page
21
Sathyabama University
Pigment Volume Concentration (P.V.C.) is an important property of paint. The higher the
volume of P.V.C., the lower will be durability, adhesion, consistency and glossy of the paint.
The following equation is used to calculate the P.V.C.
P.V .C. =
V1
V1 + V2
Where V1 = Volume of pigment in paint
V2 = Volume of non-volatile vehicle in the paint.
Characteristics of a good paint
(i)
It should have a good spreading power. (Maximum area of the surface should be
covered by minimum quantity of the paint).
(ii)
It should form durable, tough and resistant to wear film on drying.
(iii)
(iv)
(v)
(vi)
Colour of paint should not fade or change.
It should not crack on drying.
It should dry quickly.
It should give a smooth and pleasing appearance.
Mechanism of Drying of Drying Oils
Drying oils are glyceryl esters of high molecular weight fatty acids. They are the main film
forming agent of a paint. They are the binder of all other ingredients of paint. Examples are
Linseed oil, Tung oil, etc.
The mechanism of drying of drying oils involve oxidation followed by polymerization and
condensation. The structural arrangements required for drying of drying oils are
(i)
Conjugated oil structure (Example: Tung oil)
─ CH ═ CH ─ CH ═ CH ─ CH ═ CH ─
(ii)
Non-conjugated oil structure (Example: Linseed oil)
─ CH ═ CH ─ CH2 ─ CH ═ CH ─
Time required for drying of oils at 250C under standard conditions
Linseed oil
(Non-conjugated)
Tung oil
(Conjugated)
120 Hrs
(Without drier)
8 – 72 Hrs.
(Without drier)
3 Hrs.
(With drier)
1 ½ Hrs.
(With drier)
Mechanism of drying of conjugated oils
Conjugated oils contain higher proportion of fatty acids with triple unsaturation. They have
more positions (double bonds) for oxidation (O2 attack).
The absorption of O2 at the site of the double bond gives 1, 2- diradical.
Page
22
Sathyabama University
─ CH ═ CH ─ CH ═ CH ─ CH ═ CH ─ + O2
6
5
4
3
2
1
─ CH ═ CH ─ CH ═ CH ─ •CH ─ CH ─
1, 2- diradical.
│
O ─ O•
Rearrangement of double bonds in the above diradical can give 1, 4 and 1, 6- diradicals.
─ CH ═ CH ─ CH ═ CH ─ •CH ─ CH ─
│
1, 2- diradical.
O ─ O•
─ CH ═ CH ─ •CH ─ CH ═ CH ─ CH ─ and
1, 4- diradical.
│
O ─ O•
─ •CH ─ CH ═ CH ─ CH ═ CH ─ CH ─
1, 6- diradical.
│
O ─ O•
Oxygen attack at the radical site gives peroxide radical.
─ CH ═ CH ─ •CH ─ CH ═ CH ─ CH ─ + O2
1, 4- diradical.
│
O ─ O•
•
O─O
│
─ CH ═ CH ─ CH ─ CH ═ CH ─ CH ─
│
peroxide radical
O ─ O•
The peroxide radicals produce polyperoxides by polymerization, which is a durable, hard,
and adherent film.
Polymerization
Peroxide radicals
Polyperoxide (durable, adherent film).
Mechanism of drying of non-conjugated oils
The reaction between oxygen and non-conjugated oils takes place without loss of
unsaturation. That is the methylene group present between the two double bonds is prone
to direct oxidation and this reaction produces hydroperoxides.
Page
23
Sathyabama University
─ CH═ CH ─ CH2 ─ CH ═ CH ─ + O2
─ CH ═ CH ─ CH ─ CH ═ CH ─
│
O─O─H
Hydroperoxide - ROOH
This hydroperoxide decomposes to give free radicals under the influence of heat and light.
ROOH
RO• +
•
OH
The new free radical may rearrange to give a conjugated system of double bonds.
RO• + ─ CH═ CH ─ CH2 ─ CH ═ CH ─
ROH + ─ CH═ CH ─ •CH ─ CH ═ CH ─
Reaction between oxygen and these conjugated systems gives peroxide radicals, and then
polyperoxide, a durable, adherent film.
PART – A QUESTION
1. Define corrosion.
2. State the Pilling – Bedworth rule.
3. What is dry corrosion?
4. Fe corrodes faster than Al even though Fe is placed below Al in the electrochemical
series. Why?
5. What is electrochemical corrosion?
6. Mention the conditions at which electrochemical corrosion occurs.
7. Mention the catodic reactions taking place in electrochemical corrosion.
8. Define galvanic corrosion.
9. A steel screw in brass marine hardware corrodes. Give reason.
10. What is meant by differential aeration corrosion?
11. Define pitting corrosion.
12. Define microbial corrosion.
13. Iron corrodes under a drop of water or salt solution. Explain.
14. What is meant by galvanic series?
15. Distinguish between chemical corrosion and electrochemical corrosion.
16. Distinguish between electrochemical series and galvanic series.
17. A small steel tap fitted in a large copper tank undergoes severe corrosion. Justify.
18. What is meant by cathodic protection?
19. Define sacrificial anodic protection.
20. Define impressed current cathodic protection.
21. What are corrosion inhibitors?
Page
24
Sathyabama University
22. What are vapour phase inhibitors? Give an example.
23. What are protective coatings?
24. What are paints?
25. What are the various ingredients present in paints?
26. What are the functions of thinner in paint? Give examples?
27. Write a note on vehicle.
28. Give two examples of fillers in paint.
29. Mention the functions of fillers.
30. Define pigment volume concentration.
31. Write the characteristics of a good paint.
PART – B QUESTION
1. Compare the galvanic series with the electrochemical series.
2. Explain the mechanism of oxidation corrosion.
3. Explain the mechanism of electrochemical corrosion.
4. Distinguish between chemical corrosion and electrochemical corrosion.
5. Discuss the factors influencing the rate of corrosion by the nature of the metal.
6. Discuss the factors influencing the rate of corrosion by the nature of the environment.
7. Write a short note on galvanic corrosion and microbial corrosion.
8. Write a note on corrosion control by material selection and design.
9. Describe the cathodic protection method of corrosion control.
10. What are corrosion inhibitors? Explain the various inhibitors.
11. Write a short note on differential aeration corrosion.
12. What are electrochemical cells? How are they represented?
13. What is Pilling Bedworth rule? Discuss the role of nature of oxides formed in
oxidation corrosion.
14. Explain the sacrificial anodic protection method of corrosion control.
15. What is galvanic series? Explain its significance.
16. Explain in detail the various constituents present in paint.
17. Explain the mechanism of drying of drying oils in paints.
REFERENCES
1. Jain P.C. and Monica Jain,Engineering Chemistry, 15th Edition Dhanpat Rai
Publishing Co. 2009
2. Dara S.S., Text Book of Engineering Chemistry, S.Chand & Co, 2008.
3. Sheik Mideen A., Engineering Chemistry (I & II),13th Edition, Shruthi Publishers,
2010
Page
25
Sathyabama University
4. Kuriakose J.C. and Rajaram J., Chemistry in Engineering and Technology",. Vol.1 &
2, 5th reprint, Tata McGraw Hill Publishing Company (P) Ltd., 2010.
5. Sharma B.K., Engineering Chemistry, 2nd Edition, Krishna Prakasam Media (P) Ltd.,
2001.
6. Metallurgical and Ceramic Coatings edited by K.H.Stern
7. Winston Revie. R. and Herbert H. Uhlig, Corrosion and Corrosion Control An
Introduction to Corrosion Science and Engineering , 4th Edition, A John Wiley & Sons,
Inc., Publication, 2008.
Page
26